Wikipedia says damage is proportional to axle weight to the 4th power: https://en.wikipedia.org/wiki/Vehicle_weight#cite_ref-13
To give an idea of what this means:
For a 50 tonne semi-trailer with five axles, (⁵⁰⁄₅)⁴ = 10,000 units of damage.
2 tonne vehicle with two axles, 1 unit of damage.
That one truck, only 25 times as heavy as a pretty heavy car, is doing 10,000 times as much damage to the road.
Thus indeed if you have basically any heavy freight at all on a road, the rest is negligible.
It could be that four tires on a truck axle are much more effective at spreading the weight than two car tires. Or it could be that it's not much better, as the forces are still placed on a relatively small area of asphalt.
How this makes cars and trucks compare is, as far as I could understand from the literature when I dug in to it, an untested open question.
There could be a 10,000 factor difference. Or if two tires at each end are spreading the weight over 2x the surface area and you assume that matters, then you have to divide that 10,000 by 2^4 and it's now only 625x as damaging.
I suspect the reason nobody cares enough to test this is that it would still be a gap large enough to make cars insignificant.
> I suspect the reason nobody cares enough to test this is that it would still be a gap large enough to make cars insignificant.
That seems likely, yes.
My main point though is just that the study happened to use truck axles as a measurement and that can't blindly be substituted for car axles. The back of the napkin math is just there to show the difference it can potentially make.
Passenger off-road vehicles with knobby tires might be harder on roads too in a different way than weight.
It would make a significant difference to the calculations, eg 50% wider tires would mean dividing truck severity by 5.
So, getting people out of their cars onto bicycles cuts road maintenance cost.
Because of that, building bike lanes/paths can be cheaper than not building them.
(And 50 tonne trucks are very rare on most city roads, I hope)
As a Michigander, the dictionary does not contain words to express how strongly I disagree with that implication.
As a pedantic side note, this is why your batteries die, the copper leads begins to fatigue and fracture due to stresses from thermal cycling.
The principle that shifting traffic from cars to bikes saves money for the government is a sound one. There'll always be a need for cars for some things -- probably not gonna have ambulance bikes, and of course deliveries for anything big needs a car/truck -- but you can certainly reduce the need. Especially with ebikes making biking more convenient and accessible.
But not every journey needs to be done by car.
The point isn't to individually shame unwilling people out of their cars. The point is to make biking a real, viable transportation mode for short/medium distance trips for most people in urban or suburban areas. It's entirely possible to do this with the right infrastructure.
Once you do that, people will choose biking of their own accord, because it makes sense.
That isn’t an original idea. See for example https://www.vabike.org/vehicle-weight-and-road-damage/ (2009)
Also, if in Google “50 ton 5-axle truck” I get trucks that I rarely see on roads in Europe. Reading https://www.jpisla.es/resources/Download+JPIsla+20130106+Pes..., that’s because t most countries don’t allow them on roads.
Add an empty trailer and you get 35 tons (5 axles, 2400 units of damage). US max allowed is 80 tons, for a whopping 65,536 units of damage.
I don't know how full they are, but I see 5-axle tractor-trailers on highways all the time in the US, and also locally doing last-mile deliveries to larger businesses like supermarkets and home improvement stores.
I guess the differences in allowed weights might account for why US roads are often in worse shape than many in Europe, though I assume vastly different maintenance schedules play a large part as well.
[0] https://www.tcsfuel.com/blog/truck-weight-classification/
Max US ordinary large truck loading is 80k lbs, or 40 tons.
(Source, I trained as a civil engineer. Units in us practice were all over the place, but kip was generally the one that got used the most.)
https://en.wikipedia.org/wiki/Large_goods_vehicle
https://assets.publishing.service.gov.uk/government/uploads/...
As long as the roads are built to sustain those trucks the cars are negligible. So until cars are the big wear item, moving people to bikes is pointless from a road wear perspective.
Racetracks don't carry freight, but the surface does goes bad in a couple decades.
There's many more passenger cars than freight trucks on the roads.
It doesn't change the math all that much, but the 1-to-1 comparisons in the thread didn't account for this factor.